The prior art provides various methods for forming ceramic tiles for architectural applications, and also provides various glaze compositions which are used to produce the top surface or wear surface of the tile. Generally, a raw tile, sometimes referred to as a green or unfired ceramic body, is formed from natural or synthetic raw materials such as clays, kaolin, feldspars, wollastonite, talc, calcium carbonate, dolomite, calcined kaolin, oxides such as alumina, silica, corundum and mixtures of the foregoing. Binders and other additives may also be employed to increase the raw mechanical strength of the body. Once formed, the body is dried.
In many instances, a glaze composition is applied to the body either before (single firing) or after (double firing) the body is fired. Upon firing, the body forms a biscuit which is hard, tough and brittle, and the glaze composition forms a vitreous, transparent or opaque surface that can have a glossy or dull (matte) appearance. Glazes generally comprise one or more glass frits, fillers, pigments and other additives which either vitrify or sinter, depending upon the composition of the glaze and degree of firing.
For some architectural applications, it would be advantageous if the surface of a glazed ceramic tile could exhibit excellent scratch resistance and glossiness. Scratch resistance is typically measured on the Mohs Scale, which is an empirical scale of hardness based upon the ability of a surface to resist scratching when abraded by a material of known hardness, such as by feldspar (Mohs 6) or quartz (Mohs 7). For example, if the surface of a glazed tile resists scratching when abraded with feldspar, the surface is said to have passed at the Mohs 6 level.
Glossiness refers to the specular reflection of light off the surface of a material. Glossiness varies according to the incident angle of the light, and thus comparisons of glossiness must be made based upon measurements taken at the same incident angle. Glossiness measurements are reported in terms of the amount of light reflected relative to a defined standard mirror material which is considered to reflect 100% of the incident light.
Prior art glaze compositions have been able to produce ceramic tiles which exhibit excellent scratch resistance (hardness) or glossiness, but not both. Glossy ceramic tiles presently available in the market typically exhibit scratch resistance (hardness) of Mohs 4 or 5. Scratch resistant ceramic tiles (Mohs 7 or 8) presently available in the market have matte (non-glossy) surfaces and in some cases appear very rough, exhibiting a stone or cement-like appearance. A glaze composition is desired that can produce ceramic architectural tiles which have both excellent scratch resistance and glossiness.